In conventional digital electronics packaging, each integrated circuit chip is enclosed in its own hermetic package, which is then mounted on a printed wiring board by soldering. One kind of module employs approximately one hundred of these single chip packages soldered to printed wiring boards bonded on both sides of an aluminum heatsink. Instead of individually packaged integrated circuits, a current packaging method is to enclose approximately forty integrated circuits on an interconnect substrate within a single hermetic package to form a hybrid. Four of these large hybrids are then soldered to the printed wiring board modules in the same way as the conventional single chip hermetic packages. The interconnect substrates within the hybrid provide chip-to-chip connections on 0.004 inch centers instead of the 0.025 inch centers on the printed wiring board.
While printed wiring boards could be used, conventional printed wiring boards generally do not have the required density to connect silicon die that have I/O pads on 0.004 inch centers. A preferred method is generally referred to as "high density multichip interconnect." Other methods could be used and, in fact, an advantage of the hermetic module is that different interconnect methods could be used within the same module to maximize cost vs. density required.
Large hermetic packages have been used in microwave applications. The packages are usually made of Kovar to match the temperature coefficient of glass feedthroughs. Kovar is very heavy, expensive, difficult to machine, and has poor heat transfer. Aluminum is a better material, but its large temperature coefficient of expansion presents a problem when ceramic or glass insulators are used to provide Input/Output connections. Such may be useful for microwave applications, but digital electronic signal systems require a very much larger number of Input/Output connections. In the E-size standard electronic module, the number of Input/Output connections does not allow the use of individual glass beads. The number of Input/Output connections required is not feasible in this package size.